Microwave composition analysis instruments are finding increasing utility throughout industry to solve a wide range of measurement problems. Such instruments have been demonstrated to improve process efficiency, enhance product quality, and to allow automatic control of process parameters in food, chemical, petroleum, pulp and paper, and a host of other material processing and monitoring applications.
Industrial sensors and composition analyzers are used to obtain information concerning the composition of a substance to be processed by analyzing the response of the substance to electromagnetic energy. This response depends upon the constitutive electromagnetic parameters of the individual component materials comprising the substance. These parameters comprise permittivity, permeability, and conductivity. The sensors usually do not compute the actual values of the electrical parameters of each component material comprising the substance, but instead, the sensors are calibrated to yield a composition reading based upon measured signal properties that directly depend upon the electrical characteristics of the composite substance.
It is well known that the constitutive electrical properties of materials, that is, the complex electrical permittivity and the magnetic permeability, are frequency dependent. Thus, information concerning the composition of the substance can be obtained by exposing the substance to be analyzed to different frequencies and analyzing the response at each frequency. Typically, the method for measuring the frequency-dependent characteristics of materials involves sequentially generating each frequency of interest, exposing the substance to energy generated at each frequency, measuring at each frequency a response of the substance to the energy to which it is exposed, and then analyzing some aspect of the response of the material to determine the desired parameter value at each of those frequencies.
For example, a well-known technique for determining the complex permittivity of a material is to use an open-ended coaxial transmission line brought into direct contact with the material under test. A signal is applied to the transmission line and a reflection measurement is made at a number of frequencies. A permittivity value is then computed for each frequency, based upon the known characteristics of the transmission line and the reflection properties as a function of the material's permittivity as governed by Maxwell's equations.
In U.S. Pat. No. 5,331,284, Jean, et. al., describe a meter and method that is presently being marketed as a guided microwave spectrometer (GMS) system that uses a different approach for obtaining frequency dependent information. In the GMS system, a broad-band measurement is performed by stepping sequentially through a range of frequencies and measuring the transmission cutoff characteristics of a waveguide that contains the material under test. By analyzing this spectral response of the waveguide, the effects of the frequency-dependent electrical properties can be calibrated to yield multi-component analysis of various mixtures.
A host of microwave composition analyzers are on the market at present. Among those available are instruments that measure amplitude, phase, time of flight, cutoff frequency characteristics, oscillator load pull, multiple path interference, or tuned circuit resonance, either singularly or in various combinations. There are sensors that make use of reflection or transmission or both. Some instruments employ multiple frequencies or other multi-dimensional measurements of signal properties to provide multi-component analysis for certain types of mixtures.
For further background of microwave composition measurement technologies and for an extensive, though not exhaustive, list of applications for such technology refer to Jean et al., U.S. Pat. No. 5,331,284. The present invention may be advantageously employed in such applications therein listed, and included here by reference. For each of the technologies represented by the above list and those identified by reference, certain common problems exist: the microwave components comprising the instrumentation are expensive, the instrumentation lacks flexibility, or does not provide adequate performance.
In a related field of measurement, that of microwave radar tank level gauging, a breakthrough technology has emerged. Microwave level gauging radars are now available that employ Ultra-Wide-Band (UWB) pulse technology. UWB pulses are inexpensive to generate and receivers are available that allow precise distance measurement at low cost while consuming very small levels of electrical power. Application of UWB pulse technology has not been limited to radar distance measurement. UWB technology has also been identified as useful for dielectric constant measurement through time-of-flight measurement. See, for example, “Microradar Sensors for the New Millennium” available from the website of McEwan Technologies, LLC, which may be found at the following URL address: http://www.getradar.com/patents.shtml.
A somewhat unrelated yet important field of use for ultra-wideband pulses recently to emerge is that of wireless communications. Although the communications field of use is not directly related to the measurement applications addressed here, these measurement applications can directly benefit from the technology being developed for communications purposes. Wireless applications have grown to the point that at least one company has been formed for the express purpose of exploiting this technology and is developing integrated circuit chipsets to generate and process UWB pulses (“PulseON© Time Modulated Ultra-Wideband For Wireless Applications”, Time Domain, 7057 Old Madison Pike, Huntsville, Ark. 35806).
The advantage of UWB communication methods is that a limited frequency spectrum resource can be made available to multiple users without interference. The methods used to avoid interference among multiple communication channels can be applied to process control measurement devices as well, even though the uses of the spectral content of the signals may be quite different from that of communication between personal telephones or computers.
Process engineers and users of composition analysis instruments are no longer doubtful of the benefits that attain by applying microwave technology to difficult measurement situations. Today the primary impediment to the widespread application of microwave sensing instruments remains their high cost, lack of flexibility, and performance limitations. Accordingly, there is a need for an invention that overcomes these and other limitations of the prior art.